A. Field of the Invention
The invention relates generally to scheduling of unicast and multicast packets and cells, and, more particularly, to a packet and cell switching apparatus where the output units schedule unicast and multicast traffic via a credit mechanism, and where the switch unit schedules multicast traffic at a fabric scheduling stage, and where unicast and multicast traffic is merged on a per output port basis via a credit mechanism.
B. Description of the Related Art
A generic packet switching apparatus can be partitioned into the following three units: the input unit, the switch unit, and the output unit. As a packet traverses the switching apparatus from an input port to an output port, it passes these units in the listed order of sequence.
The majority of the switching apparatus' buffer capacity is typically implemented at the input unit. The input buffering typically includes Virtual Output Queue (VoQ) structures. In addition to the buffering performed by the input unit, the switch unit may also implement buffering, and the output unit may also implement buffering, depending on the implementation.
Any scheduling process can be viewed as two stages of scheduling. One stage, called “traffic scheduling”, determines the order and sequence in which the incoming packets are transferred from the input to the output port of the switching apparatus. The second stage, called “fabric scheduling”, is the level of scheduling which performs the transfer of the packets from the input to output units. In conventional cell and/or packet switching systems, both the traffic scheduling and fabric scheduling are performed by the switch unit. There is a desire to have some of those scheduling processes decentralized from the switch unit, in order to lessen the burden on that component and to distribute it to other components of a switching apparatus that have resources available to perform those processes.